KARACHI, PAKISTAN—Gunmen shot dead six health workers on an anti-polio drive in a string of attacks in Pakistan over 24 hours, officials said Tuesday, raising fears for the future of efforts to eradicate the crippling disease in one of its last strongholds.

It was not clear who was behind the shootings but Taliban insurgents have repeatedly denounced the vaccination campaign as a Western plot. The campaign is hoping to wipe out polio in one of the last three countries where it is endemic.

“Such attacks deprive Pakistani’s most vulnerable population — especially children — of basic life-saving health interventions,” said a joint statement by the World Health Organization and UNICEF, which are working with the Pakistani government on the campaign.

Health officials suspended the campaign in two provinces: Sindh and Khyber Pakhtunkhwa. Karachi, the capital of Sindh, is Pakistan’s biggest city and home to 18 million people.

Four people were killed in separate attacks on health workers in Karachi on Tuesday, said the United Nations. Another health worker was killed there Monday.

The team had received telephone calls warning workers they would regret helping the “infidel” campaign against polio, said health official Gul Naz, who oversees the project in the area where the four women were shot.

In the northwestern city of Peshawar, gunmen on a motorbike shot a 17-year-old girl supervising an anti-polio campaign, said government official Javed Marwar.

She died of her wounds in hospital, a doctor said.

All of the victims were Pakistanis who are among the tens of thousands working with a UN-backed program to eradicate polio, which can be prevented but not cured, and can cause permanent paralysis within hours of infection.

Pakistan, neighbouring Afghanistan and Nigeria are the only three countries where polio is still endemic, and so are crucial to the campaign to eradicate the disease worldwide. At least 35 children have been infected in Pakistan this year.

There have been at least three other shootings involving polio eradication workers this year.

Some extreme Islamists and Muslim preachers say the polio vaccine is a Western plot to sterilize Muslims. Other religious leaders have taken part in campaigns to debunk that nonsense.

Accusations that immunization campaigns are cover for spies were given credence when it emerged that the United States had used a Pakistani vaccination team to gather intelligence about Osama bin Laden.

In Karachi, provincial Health Minister Saghir Ahmed said the government had told 24,000 polio workers it was suspending the anti-polio drive in Sindh province.

Officials could not confirm if all the attacks were linked to the health campaign, said Michael Coleman, a spokesman for UNICEF.

Many of the attacks occurred in areas notorious for gun violence but the situation was a worry, he said.

“We’re concerned for the safety of front-line workers. They are the true heroes,” he said.